Microfluidic Device for Sorting Out Droplets

ABSTRACT

A micro-fluidic device is provided to sort out objects from a liquid stream. The device comprises a first channel comprising a first liquid and a second channel comprising a second liquid and the first liquid, and a third channel. The second channel is connected to the first channel and the channels are positioned such that a jet flow coming from the second channel can deflect objects in the first liquid into the third channel. The first liquid is a liquid which has a higher viscosity than water and the second liquid may be the same as or different from the first liquid. The micro-fluidic device is adapted for generating the jet flow in the second liquid.

FIELD OF THE INVENTION

The invention relates to the field of micro-fluidic devices to sort outobjects from a liquid stream. More specifically it relates to amicro-fluidic device to sort out objects from an oil stream.

BACKGROUND OF THE INVENTION

One increasing trend in the field of microfluidics is droplet fluidicsfor rare bio-analyte (molecules or cells) analysis. Rare target cellsare very difficult to detect or capture because they are surrounded bymany more irrelevant molecules. In droplet fluidics, individualmolecules are encapsulated in a single water-based droplet. As a result,the signal of every droplet becomes either fully positive “1” (fordroplets containing the target molecule) or fully negative “0” (fordroplets containing irrelevant molecules). In other words, the signalbecomes “digital”. In practice, droplet fluidics is created by segmentedflow, by mixing oil and water at controlled flow rates to generatethousands of microdroplets per second that can reach the picolitervolume size. Afterwards, multiple reactions can take place on alldroplets simultaneously in the same vessel. In this way, several teamsand companies have successfully demonstrated the capability of dropletfluidics for rare molecule or cell analysis. However, many applications,such as cancer therapeutics, demand isolation of the “positive” dropletsfrom the rest for downstream analysis such as DNA sequencing. This needhas been addressed by several attempts such as dielectrophoretic dropletsorting.

A prior art sorting system may for example comprise encapsulation ofsingle yeast cells from a mutant library in droplets together with afluorogenic enzyme substrate followed by sorting of droplets based onthe fluorescent signal produced by digestion of the substrate by thetarget enzyme. The encapsulation of the cell in the droplet links thecell phenotype (secreted enzyme) to genotype (yeast cell) and thefluorogenic substrate enables measurement of the enzyme concentration.Sorting of droplets is done by flowing the droplets past the sortingjunction and measuring the fluorescence of each droplet which passes afluorescence exciting laser. If the droplet fluorescence exceeds apredefined threshold, a powerful electric field is automaticallyactivated pulling the droplet of interest to a separate outlet. The cellin the sorted droplet can subsequently be recovered for furtheranalysis. Such a system can for example have a sorting rate of about 400Hz.

Prior art droplet sorting is for example done by an electrode next tothe channel. When a cell is optically detected an AC voltage is appliedto the electrode so that an electric field is created in the channel.This electric field can thereby influence the movement of the droplet.

There is still room for improvement in techniques for sorting dropletsespecially with regard to the sorting performance (e.g. the sortingspeed).

SUMMARY OF THE INVENTION

It is an object of embodiments of the present invention to provide adevice which can sort out objects from a liquid stream wherein theliquid stream is an oil stream. The objects may for example be dropletsin a sequential droplet flow.

The above objective is accomplished by a method and device according tothe present invention.

In a first aspect embodiments of the present invention relate to amicro-fluidic device to sort out objects from a liquid stream. Thedevice comprises

-   -   a first channel comprising a first liquid, a second channel        comprising a second liquid and the first liquid, and a third        channel,    -   wherein the second channel is connected to the first channel,        and wherein the channels are positioned such that a jet flow        coming from the second channel can deflect objects in the first        liquid into the third channel,    -   wherein the first liquid has a higher viscosity than water and        wherein the second liquid may be the same as or different from        the first liquid and wherein the micro-fluidic device is adapted        for generating the jet flow in the second liquid.

The jet flow in the second liquid may be generated hydrodynamically,using a MEMS device, using a heater, using external pneumatic pressurepulses, using piezoelectric actuation, or using any other means suitablefor generating the jet flow in the second liquid.

It is an advantage of embodiments of the present invention that itenables to deflect objects in a stream of a first liquid which has ahigher viscosity than water. Such a first liquid stream may for examplebe an oil stream and such an object may for example be a droplet whichencapsulates a molecule, a cell, or a particle (e.g. a sub-cellularstructure such as an exosome). The droplet is present in the firstliquid. Typically bio-analyte encapsulating water droplets are immersedin oil. It is therefore advantageous that a jet flow can be created inan oil stream which enables to sort out the droplets from the oilstream. A micro-fluidic device according to embodiments of the presentinvention is especially useful in applications where rare biomoleculesor cells (e.g. less than 1%) are targeted to be isolated for analysis.

It is an advantage of embodiments of the present invention that anoil-water interface of water droplets immersed in oil allows thedroplets to be conveniently detected with optical means, e.g. based onthe detection of a droplet edge.

It is an advantage of embodiments of the present invention that objectsin the first liquid can be sorted out from the liquid stream. This canbe done by generating a jet flow in the second liquid to deflect theminto a third channel. The objects may for example be water droplets andthe first liquid may for example be oil.

In embodiments of the present invention the micro-fluidic devicecomprises a heater adapted for generating at least one microbubble inthe second liquid for generating the jet flow from the second channel.

It is an advantage of embodiments of the present invention that theheater is adapted for generating at least one microbubble in the firstliquid causing a jet flow from the second channel which deflects objectsin the first liquid into the third channel. This enables to selectobjects in the liquid stream in the first channel by selectivelygenerating a jet flow. It is an advantage of embodiments of the presentinvention that the volume expansion caused by the microbubble produces ajet flow which allows to sort out an object from the liquid stream inthe first channel. It is an advantage of embodiments of the presentinvention that miniaturization of the microfluidic device allowsparallelization and multiplexing.

In embodiments of the present invention the first and second liquid arethe same.

In other embodiments according to the present invention the secondliquid is different from the first liquid. In these embodiments theinterface between the first and the second liquid is present in thesecond channel.

It is an advantage of embodiments of the present invention that theinterface between the first and second liquid is in the second channel.Thereby it can be avoided that the second liquid leaks into the firstchannel. The micro-fluidic device may be designed such that theinterface stays in the second channel when generating a jet flow in thesecond liquid.

The second liquid may have a lower boiling temperature than the firstliquid. When such embodiments comprise a heater, the heater may beimmersed in the second liquid. It is an advantage of embodiments of thepresent invention that the device is adapted for generating the at leastone microbubble in a second liquid which has a lower boiling point thanthe first liquid. It is thereby an advantage that the amount of heatrequired to generate the at least one microbubble is not as high as whenthe heater is immersed in the first liquid (e.g. oil). It is thereforealso advantageous that the sorting rate can be increased when using asecond liquid with a lower boiling temperature than the first liquid.The second liquid may for example be water. It is an advantage ofembodiments of the present invention that the power of the microbubblesis coupled to the first liquid via the interface between the first andthe second liquid. When the microbubbles are generated they push theinterface towards the first channel resulting in a jet flow produced bythe microbubbles. It is an advantage of embodiments of the presentinvention that the power generated by the microbubbles is conserved inthe pathway because neither the first liquid nor the second liquid arecompressible. In embodiments of the present invention the second channelis an elongated channel. In embodiments of the present invention thischannel is half filled with the first liquid and half filled with thesecond liquid. In embodiments according to the present invention thesecond channel is dimensioned such that water is not lost into the firstchannel when pushing the interface by means of the microbubbles. It isthereby advantageous that the interface remains stable over a pluralityof strokes. It is therefore advantageous that more reproducible sortingof the object is possible when using embodiments according to thepresent invention compared to for example electrostatic droplet sorting.It is an advantage of embodiments of the present invention, where thesecond liquid is different from the first liquid, that the sorting isless dependent of the channel material or droplet/first liquid (e.g.oil) composition of the first channel than in case electrostatic dropletsorting is applied.

In embodiments according to the present invention the device comprises amonitor for monitoring the interface between the first and the secondliquid.

It is an advantage of embodiments of the present invention that thestate of the interface is monitored by a monitor. Thus it is possible tocontrol whether and to which extent the interface returns to itsoriginal state after being disrupted by the microbubbles. Whenrepeatedly applying a stroke the interface may become blurred, resultingin a grey zone between the first and second liquid instead of a clearinterface. It is an advantage that this can be detected by a monitorbecause it allows to take measures to compensate for this blurringeffect. In embodiments according to the present invention more of thesecond liquid is inserted into the heating chamber or anywhere at theside of the second liquid to push the interface more towards the firstchannel side. In embodiments of the present invention even so muchsecond liquid may be added at the heater side that the second liquid isentering the first channel. After removing some of the second liquid theinterface lowers into the second channel. This allows to restore theinterface between the first liquid and the second liquid. It is anadvantage of embodiments of the present invention that thanks to themonitor system it is possible to avoid that the first liquid gets intothe heating chamber. By knowing the state and the position of theinterface it is possible to compensate for the loss of the interface intime. In embodiments according to the present invention the monitordetects when the first liquid enters too much into the second channel,upon detection the interface level may be restored by removing the firstliquid. The interface monitoring may be done by optical interfacedetection (imaging or refraction index detection). It may also be doneby one or multiple embedded electrodes at the interface area. Theelectrodes thereby may identify the liquid type by measuring for examplethe electrical impedance (or more likely the resistance) of the liquid.This is possible when there is a conductivity difference between the twoliquids. The first liquid may for example be an oil and the secondliquid may for example be water based. The conductivity of a water basedsolution can for example be set between 9e-5 S/m and 10 S/m, while theoil conductivity is normally lower, e.g. 2e-14 S/m for FC-40 oil.Therefore, by measuring the liquid impedance with electrodes, themonitor system can distinguish if the liquid is the water-based solution(low impedance) or oil (high impedance).

In embodiments according to the present invention the first and secondliquid are separated by a gas plug in the second channel.

It is an advantage of embodiments of the present invention that a morestable interface is obtained when having a gas plug in between the firstand second liquid than is the case when the interface is formed by adirect contact between the first and second liquid.

In embodiments according to the present invention the second channelcomprises an additional chamber wherein the gas plug is captured in theadditional chamber.

It is an advantage of embodiments of the present invention that the gasplug is trapped inside the additional chamber. The channels on bothsides of the chamber are narrower than the additional chamber. Thereforeit is difficult to squeeze the gas plug into the channels connected withthe additional chamber. It is an advantage of embodiments of the presentinvention that by carefully designing the ratio of the channel diameterand the additional chamber diameter this results in a microfluidicdevice wherein the gas plug is captured in the additional chamber. Thegas plug may for example fill half of the additional chamber.

In embodiments according to the present invention the microfluidicdevice comprises hydrophilic pillars in the chamber for defining aninterface between the second liquid and the gas plug.

In embodiments according to the present invention, the second liquid isthe same as the first liquid.

In embodiments according to the present invention, the devicefurthermore comprises a feedback loop comprising an optical detector fordetecting the edge of an object in the first channel and a feedbackmeans for providing information of the presence of an object fordetermining the generation of the jet flow in the second liquid.

In embodiments according to the present invention, the device comprisesa controller for controlling actuation signals for generating a jet,whereby the actuation signal may comprise a tapered leading edge andoptionally a tapered trailing edge. The latter results in the advantagethat the integrity of the object can be maintained, since more smoothjets are created.

In embodiments according to the present invention, the device comprisesa hydrophobic coating on one or more walls of the first channel avoidingthe aqueous phase to bind to the walls. The hydrophobic coating may be acoating generating a water contact angle higher then 100° on the wallsof the first channel. The hydrophobic coating may be aperfluorodecyltrichlorosilane (FDTS) monolayer.

It is an advantage of embodiments of the present invention that thehydrophilic pillars form a barrier for the second liquid.

In embodiments according to the present invention the microfluidicdevice comprises an additional channel adapted for controlling the sizeof the gas plug.

In embodiments according to the present invention the second liquid iswater.

When generating microbubbles the interface between the first and secondliquid is disrupted. It is an advantage of embodiments of the presentinvention that because of the properties of water and oil the interfacereturns approximately to its original state.

In embodiments according to the present invention the microfluidicdevice comprising a stabilizer adapted for stabilizing the interfacebetween the first and second liquid.

It is an advantage of embodiments of the present invention that by usinga stabilizer the interface returns even more to its original state,after disruption by the microbubbles, than would be the case without thestabilizer.

In embodiments according to the present invention the second channel isan elongated channel.

In embodiments according to the present invention the heater comprises astack of a metal layer in between a first and a second passivationlayer, wherein the stack is on top of a first layer which is asemiconductor substrate or glass layer.

It is an advantage of embodiments of the present invention that themetal layer of the heater can be used as heating material. It is anadvantage of embodiments of the present invention that this metal layeris not in direct contact with the second liquid. In embodiments of thepresent invention the second liquid is heated through the toppassivation layer (the second passivation layer furthest away from thesemiconductor substrate or glass layer).

In embodiments according to the present invention the metal layercomprises aluminium or tungsten.

In embodiments according to the present invention the first passivationlayer between the semiconductor substrate or glass layer and the metallayer is a layer with a lower thermal conductivity than the secondpassivation layer.

In embodiments according to the present invention the first passivationlayer is a SiO₂ layer and/or the second passivation layer is a SiNlayer. In embodiments according to the present invention the firstpassivation layer is a SiO₂ layer and/or the second passivation layer isa SiN+SiC layer. This may typically result in the following stack:silicon/SiO2/W/SiN/SiC. It is an advantage of embodiments of the presentinvention that the first passivation layer is a layer with a lowerthermal conductivity than the second passivation layer. This preventsheat from going to the semiconductor substrate or glass layer. As aresult thereof heat preferably goes to the second liquid and not to thesemiconductor substrate or glass side. This is particularly advantageouswhen the second liquid is an oil which has a high boiling temperatureand a low heat conductivity and slow dynamics (for example with regardto water). By increasing the heat transfer from the heater to the secondliquid also the sorting rate of the device can be increased. Inembodiments according to the present invention cooling may beimplemented by cooling down the chip substrate (e.g. silicon) from thebackside (this is the side opposite to the side of the fluidic layer).Preferably the heath pathway to the semiconductor substrate or glassside is blocked. The first passivation layer may for example be a SiO₂layer and the second passivation layer a SiN layer. It is not obvious tointroduce a SiO₂ layer into the stack of the metal layer, thesemiconductor substrate and the SiN layer, because of the differentthermal expansion coefficient of the SIO₂ layer compared to the thermalexpansion coefficient of the other materials in the stack. The advantageof SiO2, however, being that it has a low thermal connectivity. If thefirst passivation layer would be a SiN layer more heat would betransferred to the semiconductor substrate or glass layer.

In embodiments of the present invention the microfluidic devicecomprises a plurality of heaters. It is an advantage of embodiments ofthe present invention that a plurality of heaters may be used in tandemin multitude of ways, e.g. to improve a jet stream formation bycombining microbubbles formed by several heaters, to counteract a pushor pull phase (cf. infra) of a jet stream and/or to sort objects acrossa plurality of third channels.

In a second aspect embodiments according to the present invention relateto a diagnostic device for diagnosing a status of a patient, thediagnostic device comprising:

-   -   a microfluidic device, in accordance with embodiments of the        present invention, to sort out objects from a liquid stream,    -   an output device for determining a quality or quantity of the        objects and for providing an output based thereon on which a        diagnosis can be based.

In a third aspect embodiments of the present invention relate to anindustrial inspection device for inspecting a liquid flow comprisingobjects, the industrial inspection device comprising:

-   -   a microfluidic device, in accordance with embodiments of the        present invention, to sort out objects from a liquid stream,    -   an output device for determining a quality or quantity of the        objects and for providing an output based thereon for        characterizing the liquid flow.

In a fourth aspect embodiments of the present invention relate to amethod for forming a micro-fluidic device according to embodiments ofthe first aspect. The method may comprise providing a substratecomprising at least one jet flow actuator for generating the jet flow inthe second liquid; providing a structural layer over the substrate;patterning the structural layer in such a way that the structural layercomprises at least one channel exposing the at least one jet flowactuator; and providing a cover over the structural layer, the covercomprising at least one access to the at least one micro-fluidicchannel. The method may further comprise modifying the hydrophobicity ofa surface of the micro-fluidic channel. Modifying the hydrophobicity ofthe surface of the micro-fluidic channel may for example comprisechanging a hydrophilic surface (e.g. having a water contact angle of 30°or less) into a hydrophobic surface (e.g. having a water contact angleof 90° or more, such as)100°. Hydrophobic surfaces are advantageouslyuseful in making the microfluidic device more compatible with the use ofhydrophobic liquids (e.g. first liquids), such as oils.

Particular and preferred aspects of the invention are set out in theaccompanying independent and dependent claims. Features from thedependent claims may be combined with features of the independent claimsand with features of other dependent claims as appropriate and notmerely as explicitly set out in the claims.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a micro-fluidic device, wherein the firstliquid is the same as the second liquid in accordance with embodimentsof the present invention.

FIG. 2 schematically shows a microfluidic device comprising a secondchannel which is adapted for containing a first and a second liquid inaccordance with embodiments of the present invention.

FIG. 3 shows the interface position in the microfluidic device of FIG. 2when the heater fires a jet flow, in accordance with embodiments of thepresent invention.

FIG. 4 schematically shows a microfluidic device comprising a gas plugin between the first and the second liquid in accordance withembodiments of the present invention.

FIG. 5 shows a similar microfluidic device as FIG. 4. The microfluidicdevice comprises an additional channel adapted for controlling the sizeof the gas plug, in accordance with embodiments of the presentinvention.

FIG. 6 schematically shows a microfluidic device comprising a chamberwherein the gas plug can be captured in the chamber in accordance withembodiments of the present invention.

FIG. 7 schematically shows a micro-fluidic device comprising two heatersin accordance with embodiments of the present invention.

FIG. 8 schematically shows an optical signal measured on a stream ofdroplets in accordance with embodiments of the present invention.

FIG. 9 schematically shows an actuation signals for actuating a heaterin accordance with embodiments of the present invention.

FIG. 10 schematically shows different steps in a process for forming amicro-fluidic device to sort out objects from a liquid stream inaccordance with embodiments of the present invention. Any referencesigns in the claims shall not be construed as limiting the scope.

In the different drawings, the same reference signs refer to the same oranalogous elements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn on scale forillustrative purposes. The dimensions and the relative dimensions do notcorrespond to actual reductions to practice of the invention.

The terms first, second and the like in the description and in theclaims, are used for distinguishing between similar elements and notnecessarily for describing a sequence, either temporally, spatially, inranking or in any other manner. It is to be understood that the terms soused are interchangeable under appropriate circumstances and that theembodiments of the invention described herein are capable of operationin other sequences than described or illustrated herein.

Moreover, the terms top, under and the like in the description and theclaims are used for descriptive purposes and not necessarily fordescribing relative positions. It is to be understood that the terms soused are interchangeable under appropriate circumstances and that theembodiments of the invention described herein are capable of operationin other orientations than described or illustrated herein.

It is to be noticed that the term “comprising”, used in the claims,should not be interpreted as being restricted to the means listedthereafter; it does not exclude other elements or steps. It is thus tobe interpreted as specifying the presence of the stated features,integers, steps or components as referred to, but does not preclude thepresence or addition of one or more other features, integers, steps orcomponents, or groups thereof. Thus, the scope of the expression “adevice comprising means A and B” should not be limited to devicesconsisting only of components A and B. It means that with respect to thepresent invention, the only relevant components of the device are A andB.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment, but may. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner, as would beapparent to one of ordinary skill in the art from this disclosure, inone or more embodiments.

Similarly it should be appreciated that in the description of exemplaryembodiments of the invention, various features of the invention aresometimes grouped together in a single embodiment, figure, ordescription thereof for the purpose of streamlining the disclosure andaiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the claimsfollowing the detailed description are hereby expressly incorporatedinto this detailed description, with each claim standing on its own as aseparate embodiment of this invention.

Furthermore, while some embodiments described herein include some butnot other features included in other embodiments, combinations offeatures of different embodiments are meant to be within the scope ofthe invention, and form different embodiments, as would be understood bythose in the art. For example, in the following claims, any of theclaimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are setforth. However, it is understood that embodiments of the invention maybe practiced without these specific details. In other instances,well-known methods, structures and techniques have not been shown indetail in order not to obscure an understanding of this description.

Where in embodiments according to the present invention reference ismade to a stroke, reference is made to a jet flow caused by thegeneration of at least one microbubble in the second liquid.

Embodiments of the present invention relate to a micro-fluidic devicefor isolating objects in a liquid stream. The micro-fluidic devicetherefore comprises a first channel comprising a first liquid. Inembodiments of the present invention the objects are water droplets andthe first liquid may be any immiscible liquid that presents higherviscosity than the water phase at the droplet generation point to createa fragmented flow. In embodiments according to the present invention thefirst liquid may be an oil. In embodiments of the present invention theflow rate of the first liquid (e.g. oil) may be the same or higher thanthe flow rate of the water phase in order to create a segmented/dropletflow. The flow rate of the oil may for example be twice the flow rate ofthe water. The droplets may have captured one or more cells ormolecules. During operation the droplets pass through the first channel.Micro-fluidic devices, according to embodiments of the presentinvention, provide means for sorting out certain of these droplets whichare passing through the first channel.

The micro-fluidic device, according to embodiments of the presentinvention comprises a second channel which is connected to the firstchannel and a third channel. The first channel and the third channel arepositioned such that a jet flow coming from the second channel candeflect objects (e.g. droplets) which are present in the first channel,into the third channel.

For generating the jet flow, the micro-fluidic device comprises aheater. This heater is immersed in a second liquid in a chamberconnected to the second channel. The heater is adapted for generating atleast one microbubble in the second liquid. The microbubble(s) willinduce the jet flow from the second channel. In embodiments according tothe present invention a plurality of microbubbles may be required togenerate an efficient jet flow.

In embodiments of the present invention the second liquid is the same asthe first liquid and both are an oil. In these embodiments an oil vaporbubble is created by the heater. Therefore the oil must be heated abovethe boiling temperature of the oil which may for example be between150-180° C. Therefore, in these embodiments, the heater must be adaptedfor generating an oil vapor bubble. The heater may comprise a number ofhotspots which can be heated by applying a current pulse to the heater.The current pulse may for example have a duration of about half of thetime that the droplet travels through the sorting junction. Thereby thesorting junction is the T junction between the first channel and thesecond channel. The current pulse may for example have a durationbetween 5 μs and 1 ms. The current pulse height may for example bebetween 1 amp and 10 amp, for example between 1 amp and 5 amp. Thisheating of the hotspots consequently generates the vapor bubbles of thesecond liquid which on its turn induces the jet flow from the secondchannel towards the first channel. After the current pulse is turnedoff, the vapor bubbles collapse and the heater is restored to be readyfor the next sorting.

In embodiments of the present invention the heater is adapted forgenerating at least one microbubble in the second liquid. The heater maytherefore comprise a stack of layers wherein a metal layer is positionedbetween a first and a second passivation layer. The metal layer isthereby adapted for generating heat when sending a current through it.It may for example comprise aluminium or tungsten. The first passivationlayer may be mounted on a substrate (e.g. glass or semiconductorsubstrate). In embodiments according to the present invention the firstpassivation layer is a SiO₂ layer and/or the second passivation layer isa SiN+SiC layer. This may typically result in the following stack:silicon/SiO2/W/SiN/SiC. Other possible choices are: firstpassivation=SiN, second passivation=SiO2 or SiN (single material) orSiO2/SiC, SiO2/Ta, SiN/Ta.]

The second passivation layer may be closest to the second liquid duringoperation of the microfluidic device. The first passivation layer mayhave a lower thermal conductivity than the second passivation layer.Thus resulting in a heat flow towards the second liquid.

When the first liquid and the second liquid are the same oil, the oilpreferably has a low boiling temperature. The following oil compositionmay for example be used as carrier for the objects which may be waterdroplets generated on a T-junction area at the first channel (before theT-junction area between the first channel and the second channel):

-   -   FC-40 is fluorinated oil manufactured by 3-M company. The oil        has a clear/transparent fluid and presents a boiling point at        165° C. Its liquid density is 1855 kg/m3 and present a        refractive index of 1.29 and dielectric constant of 1.9.    -   An alternative is the HFE7500 Novec fluorinated oil that        presents a boiling point of 128° C. This oil is a clear fluid        with liquid density of 1614 kg/m³. Its refractive index is 1.39        and dielectric constant is 5.8.

FIG. 1 schematically shows a micro-fluidic device 100, wherein the firstliquid 115 is the same as the second liquid 125, in accordance withembodiments of the present invention. The first and second liquid may bean oil. The micro-fluidic device 100 comprises a first channel 110comprising the first liquid 115. In this example the first liquid canflow through the first channel and comprises water droplets. Some ofthese are wanted water droplets 111 and others are unwanted waterdroplets 112. The micro-fluidic device comprises a second channel 120connected to the first channel 110, and a heater 140. In this examplethe heater 140 is immersed in the second liquid 125 in a heating chamber145 connected to the second channel 120. In this exemplary embodimentthe heater 140 is adapted for heating the oil, being the second liquid.By heating the oil an oil vapor bubble can be generated. This vaporbubble causes a jet flow 127 from the second channel 120 and this jetflow 127 can deflect the wanted water droplet 111 into a third channel130 which is also connected to the first channel 110.

In yet another embodiment of the present invention the micro-fluidicdevice 100 comprises a first channel 110 adapted for flowing a firstliquid 115, a second channel 120 connected to the first channel 112 anda third channel 130, positioned such that, during operation, a jet flow127 coming from the second channel 120 can deflect objects in the firstliquid into the third channel 130. The second channel 120 is dimensionedsuch that it can contain a first 115 and a second liquid 125 such thatthe interface 150 between the first and second liquid is in the secondchannel 120 when both liquids are in rest and also when microbubblesgenerated in the second liquid 125 result in a jet flow from the secondchannel 120. The micro-fluidic device comprises a heater 140 adapted forgenerating microbubbles in the second liquid 125 during operation of themicro-fluidic device. During operation the heater 140 is immersed in thesecond liquid 125. For the design of the second channel 120 a tradeoffhas to be made for the size of the channel between a smaller channel inorder to minimize the disturbance of the interface 150 between the firstand the second liquid during a sorting cycle, and a larger channel witha smaller resistance towards liquid flow in order to increase the jetflow power. Thereby, both the width and the height have to be designed.In embodiments according to the present invention the channel width andheight of the second channel may for example be between 5 and 100 μm.

FIG. 2 schematically shows a microfluidic device 100 comprising a secondchannel 120 which is adapted for containing a first 115 and a second 125liquid in accordance with embodiments of the present invention.

FIG. 2 shows a heater 140 in a heating chamber 145 connected to a secondchannel 120 wherein the second channel is connected to a first channel110. The second channel is dimensioned such that the interface 150between the first and second liquid is present in the second channel 120(in rest and also when a jet flow is generated). The dimensioning maycomprise dimensioning the length and/or the width of the second channel.In this exemplary embodiment the second liquid 125 is water. The secondliquid is filling up the heating chamber 145 and the second channel hasboth a water segment 125 and an oil segment 120. Vapor bubbles createdin the heating chamber 145 lead to jet flow which propagates to thesecond channel and finally to the first sorting channel. During theentire jet flow resting and firing process, the water-oil interface 150remains in the second channel 120 to prevent oil from entering theheating chamber 145 or water from entering the first channel 110. Theadvantage of a microfluidic device 100 as in FIG. 2 is the low heatamount required to produce the jet flow, compared with the oil vaporbubble nucleation in the embodiment illustrated in FIG. 1. The reasontherefore being that the boiling temperature of water is usually muchlower than that of oil. As a result, the entire device is less heatedand thus the device is more durable and a higher sorting speed may berealized.

FIG. 3 shows the same microfluidic device 100 as in FIG. 2 when theheater heats the second liquid resulting in a jet flow. While FIG. 2shows the interface position 150 when the heater is at rest, FIG. 3shows the interface position 150 when the heater fires a jet flow 127.

In embodiments according to the present invention the second liquid 120and the first liquid 110 are separated by a gas plug 410 in the secondchannel 120 (the interface between the first and second liquid isthereby formed by the air plug). This gas plug may be an air plug, forexample an air bubble. The second channel 120 is thereby designed suchthat during the entire sorting process the gas plug 410 remains insidethe second channel 120.

In embodiments of the present invention wherein the first and secondliquid are separated by a gas plug, controlling of the microbubblesgeneration may take into account the dynamics of the gas plug. Thedynamics of such a system may be different because the air iscompressible. The interface dynamics may also be dependent on thetemperature. This may also be taken into account when generating themicrobubbles.

FIG. 4 schematically shows a microfluidic device 100 comprising a gasplug 410 in between the first 115 and the second liquid 125 inaccordance with embodiments of the present invention.

FIG. 5 shows a similar microfluidic device 100 as FIG. 4. Themicrofluidic device comprises an additional channel 420 adapted forcontrolling the size of the gas plug. The additional channel 420 isconnected with the second channel 120 at a position such that the secondchannel can contain a gas plug 410 in between the first liquid 115 andthe second liquid 125.

In embodiments of the present invention the gas plug 410 may be blockedin a certain position in the second channel 120 by providing dedicatedstructures in the second channel which prevent the gas plug fromentering such a dedicated structure. In embodiments of the presentinvention a slightly bigger chamber 510 may be introduced between twonarrow channels.

FIG. 6 schematically shows a microfluidic device comprising anadditional chamber 510 wherein the gas plug 410 can be captured inaccordance with embodiments of the present invention. Additionallyhydrophilic pillars 520 are present in the chamber. These hydrophilicpillars 520 may form a barrier for the second liquid 125 between theheater 140 and the hydrophilic pillars 520. A hydrophilic pillar allowsadditional surface tension to hold liquid 125 (e.g. water) and thereforeto trap a gas plug 410. Additionally pillars 530 may be present at thefirst liquid side 115. The interaction between the first liquid 115(e.g. oil) and the pillars 530 can be various, e.g. similar to water(e.g. “oil-philic”, hydrophobic) or the opposite, depending on the oilproperty.

Hydrophilic pillars may be used to trap a gas plug 410 (as illustratedin FIG. 6; see pillars 510, 520, 530) or they may be used when no gasplug 410 is used. In either case, the hydrophilic pillar surface onlyinteracts with a liquid. In the first method, the surface tension holdsthe second liquid 125 and consequently keeps the interface between thesecond liquid 125 and the gas plug 410 in the pillar array. In thesecond method, the surface tension holds the interface 150 between thefirst and second liquid in the pillar array.

In embodiments of the present invention the pillar array may have muchsmaller attractive force on the first liquid or on the gas plug comparedto the second liquid (because of surface hydrophilicity) and the pillararray may stop the second liquid up to a certain pressure level. Thispressure level is depending on the design of the pillar array.

In embodiments of the present invention the length of the pillar arrayis related to the stroke size (i.e. the displacement of the secondliquid during a stroke). This length may for example be the double ofthe stroke size. In embodiments of the present invention the interfacebetween the second liquid and the gas plug or the interface between thefirst liquid and the gas plug is residing between the pillar array andthe heater during an entire sorting cycle. If for some reason theinterfaces move too much toward the first channel, the pillar arrayfunctions as a safety valve to hold one of the interfaces.

The microfluidic device 100 comprising the additional chamber 510 in thesecond channel may be filled first by the second liquid 125 until thesecond liquid is immersing the heater and until the second channel isfilled up to the chamber (e.g. up to the hydrophilic pillars 520). Whenfilling up the second channel 120 with the first liquid 115 from theother side, a gas plug 410 will be trapped in the additional chamber 510between the first liquid 115 and the second liquid 125. In embodimentsaccording to the present invention the gas plug is preferably as smallas possible, in order to prevent that the gas plug decreases theefficient jet flow power. In embodiments according to the presentinvention the gas volume is no larger than 10 times the total volume ofall the vapor bubbles which are generated during one stroke. By properdesign of the second channel 120 and the additional chamber 510 the gasplug 410 will be captured inside the chamber 510 between the firstliquid 115 and the second liquid 125. For this design also pillars 520,530 may be introduced to improve the stability of the gas plug. Inembodiments of the present invention the design is such that the gasplug 410 does not fill the complete additional chamber 510.

In embodiments according to the present invention a sorting rate of morethan 500 objects per second, or even more than 1000 objects/second, oreven more than 2000 objects/second or even more than 5000 objects/secondcan be obtained. Depending on the embodiment, the sorting rate may behigher or lower. The sorting rate may be lower in an embodimentcomprising an air plug interface because of power dampening by the airbubble. This can, however, be compensated for by extra jet flow power.In an embodiment wherein the first and second liquid are the same (e.g.oil), more heating is required to produce bubbles and therefore also anadditional cooling mechanism may be required to increase the sortingrate by decreasing the required cooling time per jet flow cycle (i.e.per stroke).

In embodiments of the present invention the time between droplets to beisolated may be below one second, or it may even be below 100 ms, oreven below 10 ms, or even below 1 ms.

In embodiments of the present invention the microfluidic device 100comprises a plurality of jet flow actuators 140 (e.g. a heater). Aplurality of jet flow actuators 140 can advantageously be used in tandemin a multitude of ways. In embodiments of the present invention aplurality of jet flow actuators 140 can be present in a single secondchannel 120. In embodiments of the present invention the plurality ofjet flow actuators 140 in the single second channel 120 can be actuatedsimultaneously. The plurality of jet flow actuators 140 canadvantageously be used to form a combined jet flow which is morepowerful compared to a jet flow from a single jet flow actuator 140.This may for example allow a jet flow of the desired strength to beobtained faster, allowing in turn to achieve a higher sorting rate.

In embodiments, the plurality of actuators can reside on the same sideof the droplet-carrying channel 110 or can be arranged different. Thefirst case is illustrated by FIG. 7. One possible purpose may be tocompensate the push-pull flow. Putting the heaters on the same side cando the same job as well. Also, on the same or different side, multipleactuators, e.g. heaters, have the advantage of supporting sequentialfiring, or sequential jet flow, which can help smoothen the total jetflow and which can avoid over-using or over-heating a single heater thatmight lead to early heater failure (when multiple heaters are usedalternatingly, each heater will have sufficient time to cool down).

In embodiments of the present invention, a second jet flow actuator 140can be actuated out of phase with respect to a first jet flow actuator140. A jet flow in accordance with the present invention may inembodiments be composed of two phases: a push phase, when microbubblesare created and eject an outbound jet flow, and a pull phase, whenmicrobubbles collapse and retract an inbound jet flow. Either one ofthese phases may be used for sorting, dependent on the timing set by theuser. A problem may arise when two objects in the first channel 110 aretoo close to each other. In such a situation, after pushing the firstobject, the second object may for example inevitably be caught in a pullphase. As a result, the second cell may be wrongfully sorted. Toovercome this, the second jet flow actuator 140 may advantageously bepositioned and actuated such that the force created by its push (orpull) phase cancels out the pull (or push) phase of the first jet flowactuator 140.

In embodiments of the present invention the micro-fluidic device 100 cancomprise a plurality of second channels 120, each comprising at leastone jet flow actuator 140. This situation is depicted in FIG. 7. Theplurality of jet flow actuators 140 across the plurality of secondchannels 120 can for example advantageously be used to sort objects(e.g. wanted water droplets 111) across a plurality of third channels130, e.g. depending on their content as determined from an opticalmeasurement.

It will be clear that several or all of these uses for the plurality ofjet flow actuators 140 can in embodiments be combined. For example, amicro-fluidic device 100 may comprise a plurality of second channels120, each comprising a plurality of jet flow actuators 140. Some ofthese jet flow actuators may be used to sort objects across differentthird channels 130. Others jet flow actuators 140 may be combined toimprove (e.g. speed up) a jet stream formation. Meanwhile, some jet flowactuators 140 may be used to counteract e.g. an inconvenient pull orpush phase.

In embodiments of the present invention wanted 111 and unwanted 112objects (e.g. droplets) are detected and/or counted by an opticalmeasurement. In embodiments of the present invention the opticalmeasurement may comprise an edge detection. Droplet sorting canconventionally be achieved by detecting a scatter signal; either forwardscatter (FSC), side-scatter (SSC) or back scatter (BSC). Moreover, whenin the present invention the first liquid is an oil and the object is awater droplet, since the refractive index contrast between oil and wateracross the oil-water interface is considerable, it is convenientlypossible to detect droplets by simply measuring the double-edge of thedroplet. This is also the case for other objects, provided therefractive index contrast is sufficiently high. Alternatively alsoelectrical measurement can be performed for droplet detection. Theelectrical measurement may be adapted for detecting the droplets bytheir electrical impedance in contrast to oil, since water has adifferent conductivity and dielectric permittivity from oil. The edgedetection may be performed as an alternative or as an additionaldetection technique. It is an advantage of embodiments of the presentinvention that optical edge detection can be performed close to theposition where selection is to be performed, so that changes in speedhave little or no effect on the selection. The edge detection isillustrated in FIG. 8, at the top of which a stream of droplets 111being measured by a light source 600 is schematically depicted. Thebottom of FIG. 8 shows the corresponding measured optical signal 601,said signal 601 being characterized by valleys 602 corresponding to thedouble-edge.

In some embodiments, it may be beneficial to also consider the dropletintegrity during sorting. Unlike cells, which have a physicalencapsulating membrane, droplets only preserve their physical integrityby their surface tension; therefore, a droplet's structural integrity isoften weaker compared to cells. The forces to which the droplets areexposed during sorting may therefore advantageously be sufficiently mildin order not to damage or even break open the droplet during sorting.Referring to FIG. 9a , one way to moderate said forces may be to enlargethe duration T₁ of the actuation signal, while lowering the amplitude V,thereby controlling the power of the jet stream. Alternatively, as shownin FIG. 9b , an actuation signal may be used with a leading edge whichincreases towards a maximum intensity over a duration T₂. Likewise, anabrupt cut-off of the jet stream may also lead to the droplet beingexposed to excessive forces. The actuation signal can thereforeadvantageously be such that its trailing edge tapers down over aduration T₃. In embodiments of the present invention an actuation signalfor generating a jet flow (e.g. to a jet flow actuator 140) can comprisea tapered leading edge, and optionally a tapered trailing edge. Themicrofluidic device may thus comprise a controller for controllingactuation signals for generating a jet, whereby the actuation signal maycomprise a tapered leading edge and optionally a tapered trailing edge.

In some embodiments, the walls of the first channel may be renderedhydrophobic by applying a hydrophobic coating thereto. The walls thusmay be coated with a hydrophobic coating. The hydrophobic coating may befor example a hydrophobic perfluorodecyltrichlorosilane (PDTS)monolayer. It is an advantage of embodiments of the present inventionthat it may prevent droplets from sticking to the wall, thus allowing abetter selection of the droplets.

A method for forming a micro-fluidic device 100 in accordance withembodiments of the present invention is depicted in FIG. 10. Initially,a substrate 701 is provided (FIG. 10a ). The substrate 701 may be asemiconductor substrate (e.g. a Si substrate). The substrate 701 mayhave been processed to comprise one or more jet flow actuators forgenerating a jet flow in the second liquid in the final micro-fluidicdevice. These processing steps may typically be semiconductor technologyprocessing steps, e.g. CMOS compatible processing steps. Subsequently, astructural layer 702 is provided over the substrate 701 (FIG. 10b ). Thestructural layer 702 may be a conformal polymer layer (e.g.polydimethylsiloxane, PDMS). The structural layer 702 is then patterned(FIG. 10c ), e.g. using a photolithography, to form therein one or morechannels, such as the first 110, second 120 and/or third 130 channels;at least one channel providing an access to the one or more jet flowactuators. A cover 703 is subsequently provided over the structurallayer 702 (FIG. 10d ). The cover 703 may for example comprise a glasssubstrate, optionally itself covered with a bonding layer. The bondinglayer may facilitate bonding of the cover with the structural layer 702.The bonding layer may for example be made of a same material as thestructural layer 702, e.g. both being made of PDMS. The cover comprisesan access to the one or more channels in the structural layer 702. Tothis end, the cover 703 may be patterned with one or more openings 704;for example after bonding the cover 703 to the structural layer 702.Optionally, the method may further comprise modifying a hydrophobicityof exposed surfaces of the micro-fluidic device 100, such as bydepositing a hydrophobic coating 705 over said surfaces (FIG. 10e ). Ahydrophobic perfluorodecyltrichlorosilane (PDTS) monolayer 705 may forexample be deposited over the micro-fluidic device 100, e.g. by means ofa vapor deposition in an oven (e.g. at 120° C. for 120 min). Surfacesobtained in semiconductor technology may be hydrophilic in nature (e.g.being characterized by a water contact angle of 30° or less). Thesehydrophilic surface are typically less compatible with objects, such asdroplets. It is therefore advantageous to provide a hydrophobic coating(e.g. characterized by a water contact angle of 90° or more, such as100°) over these the exposed surfaces.

1. A micro-fluidic device to sort out objects from a liquid stream, thedevice comprising: a first channel comprising a first liquid; a secondchannel comprising a second liquid and the first liquid, and a thirdchannel; wherein the second channel is connected to the first channel,wherein the first channel and the second channel are positioned suchthat a jet flow coming from the second channel can deflect objects inthe first liquid into the third channel; wherein the first liquid has ahigher viscosity than water, wherein the second liquid may be the sameas or different from the first liquid, and wherein the micro-fluidicdevice is adapted for generating the jet flow in the second liquid. 2.The micro-fluidic device according to claim 1, further comprising aheater adapted for generating at least one microbubble in the secondliquid for generating the jet flow from the second channel.
 3. Themicro-fluidic device according to claim 1, wherein the second liquid isthe same as the first liquid.
 4. The micro-fluidic device according toclaim 1, further comprising a feedback loop, wherein the feedback loopcomprises an optical detector for detecting an edge of an object in thefirst channel, and a feedback system for providing information of thepresence of an object for determining the generation of the jet flow inthe second liquid.
 5. The micro-fluidic device according to claim 1,further comprising a controller for controlling actuation signals forgenerating a jet, wherein the actuation signals comprise a taperedleading edge and a tapered trailing edge.
 6. The micro-fluidic deviceaccording to claim 1, further comprising a hydrophobic coating on one ormore walls of the first channel.
 7. The micro-fluidic device accordingto claim 6, wherein the hydrophobic coating is aperfluorodecyltrichlorosilane (PDTS) monolayer.
 8. The micro-fluidicdevice according to claim 1, wherein the second liquid is different fromthe first liquid and wherein an interface between the first liquid andthe second liquid is present in the second channel.
 9. The microfluidicdevice according to claim 8, further comprising a monitor for monitoringthe interface between the first liquid and the second liquid.
 10. Themicrofluidic device according to claim 8, wherein the first liquid andthe second liquid are separated by a gas plug in the second channel. 11.The microfluidic device according to claim 10, wherein the secondchannel comprises an additional chamber, and wherein the gas plug iscaptured in the additional chamber.
 12. The microfluidic deviceaccording to claim 11, further comprising hydrophilic pillars in thechamber, wherein the hydrophobic pillars define an interface between thesecond liquid and the gas plug.
 13. The microfluidic device according toclaim 10, further comprising an additional channel adapted forcontrolling a size of the gas plug.
 14. The microfluidic deviceaccording to claim 8, wherein the second liquid is water.
 15. Themicrofluidic device according to claim 14, further comprising astabilizer adapted for stabilizing the interface between the firstliquid and the second liquid.
 16. The microfluidic device according toclaim 1, wherein the second channel is an elongated channel.
 17. Themicrofluidic device according to claim 2, wherein the heater comprises astack, the stack comprising a metal layer in between a first passivationlayer and a second passivation layer, wherein the stack is on top of asemiconductor substrate layer or a glass layer.
 18. The microfluidicdevice according to claim 17, wherein the first passivation layer has alower thermal conductivity than the second passivation layer.
 19. Adiagnostic device for diagnosing a status of a patient, the diagnosticdevice comprising: the microfluidic device according to claim 1; and anoutput device, wherein the output device determines a quality orquantity of the objects and provides an output diagnosis based on thequality or quantity of the objects.
 20. An industrial inspection devicefor inspecting a liquid flow comprising objects, the industrialinspection device comprising: the microfluidic device according to claim1; and an output device, wherein the output device determines a qualityor quantity of the objects and provides an output based on the qualityor quantity of the objects, wherein the output characterizes the liquidflow.
 21. A method for forming the micro-fluidic device according toclaim 1, the method comprising: providing a substrate comprising atleast one jet flow actuator for generating the jet flow in the secondliquid; providing a structural layer over the substrate; patterning thestructural layer such that the structural layer comprises at least onemicro-fluidic channel exposing the at least one jet flow actuator;providing a cover over the structural layer, wherein the cover comprisesat least one access to the at least one micro-fluidic channel; andrendering a surface of the at least one micro-fluidic channelhydrophobic.